This invention relates to pistons made of light alloys such as aluminum alloys as the matrix metal and finding utility in diesel engines for automobiles, and more particularly, to pistons having a cavity for air heat insulation or other purposes within their head.
Most of pistons currently used in advanced engines are those cast from light alloys as exemplified by aluminum alloys for the main purpose of achieving a weight reduction to reduce the inertia force of reciprocating parts. Since aluminum alloy, however, has a high thermal conductivity, an engine having pistons of aluminum alloy has the problem that a substantial amount of the heat generated in the combustion chamber by the combustion of fuel is conducted outside the combustion chamber through the pistons and the thermal efficiency of the engine is accordingly reduced. This results in reductions of fuel consumption and power, while leaving a risk of incomplete combustion at an initial period from the start. In recently developed engines having aluminum alloy pistons mounted, particularly diesel engines, attempts of preventing leakage of heat from the combustion chamber through the pistons by providing a piston head of heat insulating structure were made for the purposes of keeping the combustion chamber at higher temperatures to improve fuel consumption and power and preventing incomplete combustion at an initial period from the start.
One of known effective means for rendering the piston head heat insulating is to form immediately below the piston head a hollow space or cavity for containing heat insulating air. To accommodate an increase of the head temperature due to heat insulation, the head is formed from heat resistant material. More particularly, a head member formed from a heat resistant material such as a superalloy, typically Inconel is fastened to a piston body by bolts or the like while providing a cavity therebetween. This technique requires a previous step of forming holes and threads in the head member and the piston body as by machining in addition to the bolting step, and thus leads to low productivity and increased cost. There also arises a problem during the operation of the piston that the piston body, particularly at the site of bolt holes undergoes creep deformation, losing the effective bond strength between the heat resistant material head member and the body.
There is a great need for the development of a method for producing a piston having a cavity for heat insulation just below its head without the problems of cost increase and productivity decline. One method believed effective for such purposes is the application of an insert embedded casting process wherein matrix metal is cast into a piston body in which a head member of heat resistant material is incorporated as an insert while a cavity is left immediately below the head member. The effective casting processes used herein are pressure casting processes including so called high pressure casting process because casting of matrix metal with an insert embedded is facilitated and because little defects are introduced and a finer grain structure is achieved in the resulting piston body.
In most commonly used methods for creating a cavity within a casting, a casting having a sand core such as a shell core inserted therein is first formed and the sand core is then removed from within the casting. Alternative methods commonly used are by casting a part using a core of a material capable of being readily dissolved in such a solvent as water, for example, a salt core, and removing the core by dissolving away after the casting.
When a high pressure casting process is applied to cast molten metal using a sand core, the molten metal is infiltrated into the core by the high pressure applied thereto, making it difficult to remove the core sand from within the casting. A similar problem occurs with the use of salt cores. Compression molded salt cores can be impregnated with molten metal during high pressure casting. Salt cores solidified from a metal tend to develop cracks during high pressure casting.
It was thus very difficult in the prior art to form a cavity of any desired shape within a casting by pressure casting processes such as high pressure casting.
The air heat insulation layer to be formed immediately below the heat resistant material head member of a piston may be provided by a porous heat insulating layer containing a plurality of fine pores as well as the above-mentioned cavity. As opposed to the insulating layer in the form of a whole cavity, the provision of a cellular heat insulating layer in the form of a porous body is effective in preventing the heat resistant material head member from deforming under combustion pressures. This, in turn, allows the use of a thinner head member which leads to a reduction of piston weight, probably contributing to some improvements in engine performance and fuel consumption.
Prior art methods for forming a porous portion within a casting involve embedding hollow spheres such as shirasu baloons or inserting a porous body such as a shell core during casting.
If the above-mentioned formation of a porous portion within a casting by embedding hollow spheres therein is combined with the high pressure casting process, the hollow spheres are ruptured by the pressure applied to the molten matrix metal, failing to form the porous portion having the desired porosity. As for the method of directly inserting a porous body in a casting, the porous body is impregnated with the molten matrix metal under pressure to form an impregnated body having low heat insulation.
It was thus very difficult in the prior art to form a porous portion for air heat insulation having any desired shape and porosity and free of any impregnating matrix metal within a casting by pressure casting processes.
In pistons of aluminum alloy, it has been a common practice to provide a cavity or porous portion inside the side wall of the piston where piston ring grooves are formed. This cavity or porous portion serves as a cooling channel or oil gallery to cool the grooved side wall with cooling oil from the inside for the purpose of improving the wear resistance of the grooved side wall. The same discussion as above is applicable to the formation of such a cooling oil channel.